Foamed thermoplastic materials are used in many applications including, but not limited to cushioning and packaging materials. insulation, foamed extruded profiles and injection molded structural foam. Foamed thermoplastic materials offer numerous advantages over other structural materials including thermal and sound insulation, reduced weight per unit volume and low cost per unit volume.
Various methods are known for producing cellular thermoplastic articles which are generally referred to as "foam" or "foamed thermoplastic" articles. Foaming is typically accomplished by injecting an inert gas such as nitrogen or carbon dioxide under pressures ranging from about 8,000 to 15,000 psig into a molten polymer followed by a release of pressure to expand the thermoplastic material to form a foamed article. Addition of volatile liquids such as chlorofluorocarbons and pentane are commonly employed to foam specific polymers including polystyrene and various polyurethanes. Another group of blowing agents, typically referred to as chemical blowing agents, decompose upon heating to generate gas which is utilized to expand the polymer. Chemical blowing agents are commonly employed in the extrusion of profiles and foam injection molding of automotive parts. Chemical blowing agents utilized for foaming polymers include both inorganic materials ( e.g., sodium bicarbonate and sodium borohydride) and organic materials (e.g., azodicarbonamide, tetrazoles and semi-carbazides) . Azodicarbonamide, which decomposes in the range of 400.degree. to 450.degree. C., is a commonly used chemical blowing agent which generates nitrogen and carbon monoxide upon decomposition. P-toluenesulfonyl semi-carbazide and 5-phenyltetrazole, having decomposition temperature ranging from 440.degree.-450.degree. F. and 460.degree. -500.degree. F., respectively, are utilized in the foaming of higher temperature engineering polymers. Variants of 5-phenyltetrazole are noted to be useful as foaming agents in processes wherein processing temperatures approach 750.degree. F.
In the foaming of molten thermoplastic resins, volatilizable blowing agents form bubbles around nuclei within the molten polymer resin. While a certain number of nuclei are inherently present within the molten polymer, additional nucleating agents may by added to facilitate the production of fine-celled foams. Examples of materials employed as nucleating agents include surfactants, dissolved inert gases, gas-liberating thermally decomposable solids. non-decomposable inorganic solids and materials which provide hot spots such as below melting metals.
In the production of cellular thermoplastic articles wherein a molten thermoplastic resin is expanded utilizing a dissolved, soluble volatilizable blowing agent, bubble formation takes place in the melt as the pressure on the melt is reduced. Typically, such a pressure drop takes place as the dissolved foaming agent within the melt, residing at a higher temperature and pressure, passes through a die or orifice to the outside of the die or orifice which typically resides at ambient pressure and temperature. The dissolved foaming agent surrounding the bubble diffuses into the bubble as a bubble is initiated at a nucleating site. Bubble formation is facilitated by the presence of an effective amount of nucleation sites. If such sites are present, bubbles will be formed when the driving force is relatively low whereas high driving forces are required when nucleation sites are relatively few in number.
The prior art teaches various processes for forming thermoplastic articles wherein highly pressurized nitrogen is utilized as a foaming agent. Such processes typically are modelled after conventional systems known in the art which employ chlorofluorocarbons as foaming agents. More particularly, nitrogen, which is insoluble in typical thermoplastic materials at ambient pressure, is introduced at pressures ranging from about 8,000 to 15.000 psig into an extruder at a location immediately upstream of the die or orifice. Utilization of such high pressures renders the nitrogen gas soluble in the thermoplastic material. For example, U.S. Pat. No. 4,390,332 discloses an apparatus for the injection molding of parts from foamed thermoplastic materials wherein an inert gas is injected into the melt zone of the screw and barrel assembly under high pressure and the gas bubbles which are soluble in the polymer melt at the elevated pressure are mixed with the molten plastic material as a result of the rotation of the screw within the barrel member to form the foamed plastic material. The Specification ( col. 4, line 68) states that the gas is introduced under high pressure.
U.S. Pat. No. 4,473.665 discloses a method for preparing microcellular foamed materials wherein the material to be processed is presaturated with a uniform concentration of an inert gas while controlling temperature and pressure to avoid cell nucleation. Processing is also done under pressure to avoid nucleation. After processing, pressure is released and cell nucleation occurs at or near the glass transition temperature of the material. The Specification ( col. 2, lines 43-54) states that the introduction of inert gas into the barrel of an extruder provides an unsatisfactory product because it is very difficult to measure the quantity of gas introduced into the process and the mixing stage of conventional extruders is not capable of dispersing and collapsing the large pockets of nitrogen gas within the polymer melt.
Those skilled in the art of foaming thermoplastic polymers with chlorofluorocarbons have been searching for alternative foaming agents which are inexpensive, non-hazardous to the environment and which can be used with minimum modification to the equipment presently used for foaming polymers using chlorofluorocarbons as a foaming agents. Unfortunately, previous attempts have not been entirely successful because such processes typically require complex equipment and are operated under very high pressures, typically approaching 15,000 psig, in order to supersaturate the polymer to be foamed with the foaming agent which is essentially insoluble at ambient pressure. The present invention overcomes the limitations stated in U.S. Pat. No. 4,473,665 and provides a process for foaming thermoplastic materials at substantially lower pressures with minimum equipment modifications.